Electrical insulating and heat-conducting material



v UNITED STATES v ROBERT No Drawing.

To aZZ'whom-it may concern:

Be it'known that I, ROBERT W. EARLE, a citizen of the United States, resident of Chicago, county of Cook, and State of Illinois,

have invented new and useful Improvements in Electrical Insulating and Heat-Conducting Materiah'of which the following is a specification, the principle of the invention being herein explained and the best mode in which I have contemplated applying that principle, so as to distinguish it from other inventions. 1

My invention relates to electrical insulating and heat conducting material for sad irons, soldering irons, stoves and other heating, devices, such as described in my pending application Serial No. 196,610,filed October 15, 1917. v

The object of my invention is to provide a material of the above-described character from which uniform results may be obtained and the various units thereof made to conform to a maximum commercial standard of dielectric strength, heat conductivity and durability. Y

It has long been known that certainclays such, for example, as 'fireclay,-possess considerable dielectric strength. Also, that aluby the additionof water, resistance, wires' embedded therein and the unit thus formed molded to the requiredshape, compressed in a container-to secure intimate contact with the wire and the walls of thecontainer, and baked in the form of a brick, from which the ends of the wire. protrude in a position to'serve as terminals.

' Some of the units formed in this manner have been sufficiently successful for com mercial purposes. They usually possess substantially the same coefficient of expansion and contraction as that of the metal containe'rs in which they are placed. Some .of

PATENT OFFICE...

w. EARL 0F 'omcAGq'InLINoIs, ASSIGNOR To THE novER MANUFAC- TUBING COMPANY, or DOVER, OHIO, A CORPORATION or 01110.

. ELECTRICAL INSULATING AND HEAT-CONDUCTING MATERIAL.

Specification of Letters Patent. t t 10, 1922, a Application filed June 27, 1918. Serial No. 242,250.

the units have been found to possess high dielectricstrength. In some of them, the dielectric strength has been found to be so low that danger existed ofserious shock to the persons handling the metal containers,

due to current leakage from the resistance wire through the embedding material to such containers.

form in which they were baked, under conditions of varying temperature and moisture, yet in other units, apparently made sistance wire andethe container, or between successive convolutions of wire, or in other cases permitting deterioration of theresistance wire by exposing itto atmospheric conditions. Variations have also been noted in the thermal conductivity of different units.

For the above reasons the successful exploitation and commercial use of heating units of this type have been greatly jeopardized and so far as I am aware, no one has heretofore sufliciently understoodeither the processor the reasons why were obtained in some cases, failures in other cases. 3

Myinvention is based upon the discovery that the above-mentioned variations in results are due to the fact that the materials used are not properly prepared for such use and that the process followed in the manufacture of the units, as heretofore carried on, has been such that successful results have been accidental. I have discovered that it is sometimes necessary to first prepare the material by removing therefrom foreign substances, and where the latter consist of iron and othersubstances which'may serve as conductors for the electric current such removal is in all cases desirable; that it is also necessary to use distilled water or water from which current conducting salts or minerals are absent, in order to secure the best results, and also to uniformly secure high dielectric strength. I have further discovered that success is dependent upon using, within reasonable limits, definite proportions of the fused granular material and of the binding material and to subject these ollowed by It has also been found that ,while some of the'units retained the brick.

good results secure intimate contact between the material and the wire and with the walls of the container. I have also found it necessary .to' predetermine the dielectric strength of the bonding material, and to subject the material of the unit to a predetermined pressure beyond which the dielectric strength will not, decrease. Those portions of the bonding material which dissolve and produce" plasticity are thus forced into the inter sti ces between the harder grains and the ines of conduction which would otherwise extend from the resistance wire to the container through this bonding material, are thereby in a large measureeither broken or greatly extended and reduced in cross-section. In this manner the electrical resistin qualities of the grains of previously fused material are utilized to the best possible advantage.

I have further discovered that in order to secure the bestresults both as to dielectric strength and heat conductivity, it is necessary to exclude moisture from the interior of the unitas far as possible, there being a tendency for such units to absorb considerable moisture from the atmosphere. I have discovered that different clays used as. bonding materials differ in moisture exeluding qualities or effects and this is due to that portionof the clay whichis capable ofbeing dissolved or partially dlssolved in the water and held in colloidal like relation. This colloidal like clay content constitutes the real bond between the grainsv of fused bauxite when dried and baked.

In the practice of my process, I first fuse a quantity of selected highly aluminous material such as bauxite, or a :mixture of bauxite and clay. This fusion may be accomplishedin an electric arc furnace at a temperature of about five thousand (5000) or six thousand .(6000) degrees Fahrenheit or more. The material thus fused or melted forms into a crystalline mass when cooled. which is then crushed into fragments. For my process these fragments are reduced to a degree of fineness whereby of about fifty-nine ten thousandths (.0059) 55y of an inch in diameter. The material commercially known as alundum grain manufactured by the Norton Co., of lVorcester,

Massachusetts, is made; in the above descrlbed manner and I have found this to be a convenient source of supply. of this class ofmate'rial. I then remove from this material any iron or other-ingredients having marked electrical conductive qualities. The iron (or fragments containing iron) can readil'v he removed-by passing the granular or powdered material in dry form through a magnetic separator. Chemical processes for the removal of the iron and other conductive materialsma-y also be utilized for this purposeif desired. This granular ma terial when made from bauxite is substantially a fused aluminum oxide. v

I then mix with the non-magneti'zable; granular or powdered material above described, a quantity of bonding material such as a clay in order that the mass may be baked into brick form under the conditions hereinafter described. I have dis covered that the bonding material for this purpose should have the following properties, namely; high dielectric strength; a

fusing point which will not be approxi- V mated or materially approached. at any unit will be subjected when in use; capabil-' ity, when mixed with, water, ofbeing held in colloidal relation to make it plastic; and considerable heat conductivity. f ,3

By the use of materials having properties thus described, the same being prepared and fabricated in the manner set forth, I produce a ceramic body o f'relatively high dielectric strength with suflicient mechanical strength and of good heat conductivity, which will not craze or crackle orv sufl'er destruction because of unequal expansion. This insulating material, which may be, fair ricated with a resistance wire into a heating unlt, is distinguished from-the insulating mediums which have been used, such as glass, porcelain, and such natural mediums as mica, in that my insulating medium is compact without being, vitrified .a condition with dielectric strength. I

I have discovered certain grades (if 111i H015. plastic clay known in the trade as I ll1no1s kaolin, which in fact is. not kaolin,

to answer the above requirements in a satisfactory manner for my purposes; Thls clay resembles kaolin in chemical comfractory, and exhibits a surprisingly high degree of plasticity. When burned, it is not white, but has a deepbufl color, due no doubt to the influence of the iron which -may be present in a combined form, as a position, is quite fine grained and vver'y resilicate or titinate or some other stable 'c0m pound. -Undei' both oxidizin and reducing conditionsit vitrifies Ver 5 owly, due; no doubt, to the fact that it contains but a trac'e of such fluxing in redients as lime,

magnesia, and alkalies. uch clay maybe obtained in the 'vicinityof Union County, Illinois, and is being sold by the IlliIlOlS Kaolin Co., Anna Illinois. I use this clay" in finely divided form, grading it by passing it through a screen similar to that above- I described. If iron or other electrical cohductlng materials are found to be present electric strength the best results are obtained' by using about fifteen per cent (15%) of bonding material to eighty-five per cent (85%) of fused material. The bonding material being less dielectric than the fused material and not being'so highly heat-conductive, it is desirable to use as small a quantity as is consistent with the production of a durable brick, and it is also desirable that this bonding material should be pocketed as far as possible in the interstices between the fused fragments or granules. It appears that where the fine-grained or dispersed constituents of the bonding material are-present in relatively large proportions, a smaller amount of bonding material may be employed than otherwise, this being due to the fact that the bond is provided by that portion of the material which is capable of forming a colloidal like relation'when'water is added to the mixture.

Having provided the mixture abovedescribed, I add a quantity of distilled water, or water free from all foreign matter'such as salts having an material degree of electrical conductivity. ufiicient water is added and thoroughly mixed with the ma-' terial until the mass assumes a consistency whereby it may be readily worked. The pre-. cise quantity of water is not material, since" my process can be carried out, either with just sufficient water to facilitate molding orlforming the material within a receptacle, or the water may be in such quantity that themixture may be poured by means of a ladle. It is desirable, however, that the mixture; be sufliciently thick to prevent a conducting wire from settling by gravity or to prevent a crimped or convoluted wire.

from changing its position within the mixture after it has once been placed therein.

The mixture of water, fused granular material and bonding material. is then placed in a receptacle or container and a suitable resistance wire or element, such as nichrome wire, of the proper length and cross-section embedded therein, the ends of the wire hav i'ng previously been provided with conducting term nals of copper or other ourrent-conducting and comparatively non-- corrosive material, which are allowed to protrude from the surface of the mixture after the wire has been embedded.

A convenient mode of embedding the wire I is to partially fill the container with the mix;-

ture and then press the wire into it after the wire has been shaped and annealed, or if desired, the mixturefirst poured into the receptacle may be allowed to partially dry and the upper portion may then be remoistened to facilitate inserting the wlre.

The receptacle is then additionally filled with, the embedding mixture in order to completely cover all portions; o-f the wire except the projecting terminals.

After the resistance wire *has been embedded as above described, the mixture is allowed to partially dry and is then sub jected to a pressure of preferably about two and a half (24,) tons to the square inch. This pressure is calculated to be approximately the maximum pressure that the mass will withstand without reducing the dielectric strength and therefore if the character" of the fused material is varied from time to time, it may also be necessary to vary to some extent, the degree of pressure, but it appears from numerous experiments which I have made that the dielectric strength of the unit is increased by applying this heavy pressure up to a certain pointbeyond which it decreases, this point with the mixture above-described being about five thousand (5000) pounds per square inch. The reason for this has not been up to the present time established with absolute certainty but it may possibly result from stratification of the material including the bonding material as a result of this pressure, thereby pro ducing shorter continuous lines of the material of less dielectric strength and hence shorter conductors between the resistance element and the container.

After thus compressing the unit, the latter is allowed to dry at a comparatively low temperature until the greater portion of 4 the moisture has been removed and in fact until the unit appears to be thoroughly dried from a superficial standpoint. The unit is then placed in a baking oven and subjected to a temperature of about twelve hundred degrees (1200) Fahrenheit, whereby it is" baked in the form of a brick, the alundum grains being united with each other and the embedding material connected intimately and rigidly .with the resistance wire and the walls of the container.

I am aware thatsembedding materials have been subjected to pressure in order. to secure intimate contacts between the materials and the Walls of,the container and the wire. This is also accomplished in the compressing operation above described but ll believe that it has not been heretofore known that the dielectric strength ofsuch units could be increased by subjecting the material to a definite predetermined pressure. All clays contain, granular materials and it has been observed in some cases that the dlelectric strength was increased by pressure, although the reasons why. this sure. j

The Illinois plastic clay which I employ was accomplished were not understoodand this result, i. e., the increase in dielectric strength, was "not uniformly accomplished but was a result accidentally secured by an inadvertent application of sufficient presdoes not fuse to any appreciable extent at a temperature of less than three thousand degrees (3000) Fahrenheit. The temperature of the resistance element in an electric heating unit of this type, under ordinary working conditions, ranges from three hundred 300) to one thousand two hundred degrees (1200) Fahrenheit; It is therefore obvious that the fusing point of the bonding material is not suificiently approached by an ordinary working temperature to materially. .reduce its dielectric strength. The same is true in even more.

markeddegree of the fused fragments or granules, which. re liire about. three thousand five hundred 23500) degrees, Fahrenheit in order to soften or melt them-p Numerous tests which I have made have demonstrated that dielectric clays usually lose their resistance 'very slowly as the temperature is increased, until a temperature is reached within a few hundred degrees of the point where vitrification or incipient fusion becomes noticeable, after which any increase in temperature causes a rapidloss in dielectric strength. It is therefore exceedingly important in the selection of embedding materials or mixtures for embedding materials, that they be not only di- 'grees Fahrenheit. I have discovered that i this is true of different clays which I have "tested and I believe that I am the firsttol select'clays for thepurpos'e of manufacturing electrical heating units with reference to their dielectric strength while heated at temperatures to which they will be subjected in the useof such units and to provide a bonding clay. of substantially uniform dielectric strength under the varying conditions of temperature to which such units are subjected.

The fused aluminum oxide grains. or alundum grams are important. factors in providing a unit having the requisite heat conductivity and also in providing a nonshrinking body of embedding material whereby the mixture will remain in contact with the heating element and the walls of the container during the drying process. If the embedding material were composed entirelyfof unfused clay, there would be a considerable shrinking of the mass during the drying and baking processes and this would tend to destroy the intimate contacts necessary for proper heat conductivity and would alsoten'd to expose the heating element to atmospheric influence, particularly moisture, and would further tend to promote disintegration of the unit. The fused grains of aluminum oxide do not swell or shrink undervarying conditions of moisture and although the bonding material does expand, ,with the addition of moisture and shrink when dry, "yet therelatively small quantity of this material present in the embedding mixture has no appreciable effect in causing the embedding material to shrink away from the wire and from the container during the dryingand baking processes In efiect the granular material'constitutes a rigid frame held together, after the baking process, by the bonding material but sufliciently rigid under all conditions to prevent the mass from shrinking away from the points of contact with the container and the resistance WIIG;

I have referred to the mixture of embedding material as a homo eneous mixture in the sense that the hon ing material is uniformly distributed throughout the mass of fused grains, but itwill be observed that the embedding material of the completed unit is also homogeneous in another sense, namely, that of being one integral mass of material .withoutdiVIlSiOIl lines produced by inserting the material into the container in the formof layers. This is true because no hydraulic action takes-place in the mixture, after the addition of water, such as might tend to solidify the mixture or cause it to set, even initially after the first layer has been inserted in the container and before the last or covering layer is applied to cover the resistance element. Y Even if the 'first layer is permitted to dry in any degree before the second layer is applied, the

moisture contained in the second layer will permeate the surface of the first layer to some extent and when the unit is subjected the moisture will be thoroughly distributed ,to the heavy compression above-described,

throughout the entire massof embeddingmaterial. and some portions thereof will be squeezed out. This expulsion of the liquid makes it desirable that the' unit be covered with dry powder orwith a quantity of the 'dry mixture before inserting the unit into the press to avoid adhesion of the moisture to theplunger.

Having fully described my invention, what I claim and desire to secure by Letters Patent is:

1. An electrical insulating and heat-conducting material consisting of grains of high dielectric strength and thermal conductivity connected by a compact non-vitrified bonding material of high dielectric strength and high fusing point.

2. An electrical insulating and heat-conducting material consisting of grains vof high dielectric strength and thermal conductivity connected by a compactnon-vitrified bonding material having a high dielectric strength at the Working temperatures of the electrical heaters in which the material is to be used, said bonding material also having a high fusing point.

3. An electrical insulating and heat-conducting material consisting of grains free from iron and of high dielectric strength and thermal conductivity connected by a compact non-vitrified bondingmaterial of high dielectric strength and high fusing point.

I. An electrical insulating and heat-conducting material consisting of grains of high dielectric strength and thermal conductivity connected by a compact non-vitri fied bonding material of high dielectric strength and high fusing point and free from iron.

An electrical insulating and heat-conducting material consisting of a v highly compacted mass of grains of high dielectric strength and thermal conductivity connected by a' compact non-vitrified bonding material of high dielectric strength and high fusing point.

6. An electrical insulating and heat-conducting material consisting of a highly compacted mass of grains of high dielectric strength and thermal conductivity connected by a non-vitrified bonding material of high dielectric strength andhigh fusing point, the degree of compactness being such as results from a pressure beyond which a decilease of dielectric strength would take p ace.

7. An electrical insulating and heat-conducting material consisting of grains of high dielectric strength and thermal conductivity connected by a compact bonding material consisting of non-vitrified Illinois kaolin.

8. An electrical insulating and heat-conducting material consisting of grains of fused aluminum oxide connected by a compact bonding material consisting ofnonvitrified Illinois kaolin. i

9. An electrical insulating and heat-conducting material consisting of a highly compacted mass of grains of high dielectric strength and thermal conductivity connected by a compact bonding material consisting 6 of non-vitrified Illinois kaolin.

10. An electrical insulating and heat-conducting material consisting of a highly compacted mass of grains of 'high' dielectric strength and thermal conductivity connected by a bonding material consisting of non vitrified Illinois kaolin, the degree of compactness being such as results from apressure beyond which a decrease of dielectric strength would take place.

Signed by me, this 18th day of June, 1918.

R. W. EAR-LE. 

